1. Field of the Invention
The present invention relates to an acoustic gas meter and in particular to a meter for the analysis of proportions of gases in gas mixtures.
2. Description of the Prior Art
In medical and clinical settings it is useful to be able to measure accurately the concentrations and/or flow rates of respiratory (inspiration and/or expiration) gases or changes therein since these can provide, for example, valuable information on patient metabolic conditions. This is particularly the case during the provision of mechanical respiratory aid to a patient where knowledge of the relative and absolute amounts of oxygen and carbon dioxide within the expiration gas may be used to provide information on the metabolization of oxygen as well as respiratory function. Moreover, knowledge of the oxygen/nitrogen ratio in an inspiration gas is useful for controlling or monitoring the provision of mechanical respiratory aid.
Known acoustic gas meters have an ultrasonic transducer arrangement adapted to transmit ultrasound pulses along an acoustic path through a gas mixture within a measurement cell or a section of a flow conduit containing the flowing gas mixture and to receive the transmitted pulses; and a calculation unit for calculating the transit time of the ultrasound pulses. The transit time calculated in this manner may be used in known techniques to provide a measure of the flow and, additionally or alternatively, the composition of the gaseous medium. Because the velocity of sound through a gaseous medium is known to be dependent on the temperature of that medium then a temperature probe is often included as part of the meter to monitor the gas temperature at a point within the cell or section and to provide this temperature to the calculation unit where it may be employed to compensate the calculated parameters for temperature variations.
Gas meters are known, for example from PCT Application WO 92/03724 and from U.S. Pat. No. 5,247,826, for acoustically analyzing the ratios of a mixture of gases comprising two known gases, such as the oxygen/nitrogen ratio in a breathing gas to be supplied to a patient from which the oxygen concentration or changes therein can be determined. Such known meters utilize the physical phenomenon that acoustic waves travel with different velocities through different gases. The velocity of sound, V, through a gas is known to be proportional to (T/M)0.5 where M is the molecular weight of the gas and T is its absolute temperature. Thus for a gas mixture at a known temperature the velocity of sound, V, in the mixture can be used to provide a measure of the relative concentrations of the constituents of the gas.
However, the temperature of the gas will vary according to its pressure so that in circumstances where the gas has a variable and rapidly changing pressure, such as typically found in inspiration and expiration gases during mechanical respiratory aid, inaccuracies in the measured gas ratio can occur (typically a 10C error in temperature will give approximately a 3% error in oxygen concentration in a binary gas mixture with air). This is particularly problematical when the pressure induced temperature variation causes a temperature gradient to occur within the acoustic path through the gas mixture to be analyzed.
The above object is achieved in accordance with the principles of the present invention in an acoustic gas meter having an acoustic transmitter/receiver arrangement for transmitting and receiving acoustic energy along an acoustic path, and a temperature probe having a sensor region disposed to measure a gas temperature, said sensor region being elongate and disposed relative to the acoustic path to provide a measure of a gas temperature indicative of an average gas temperature within the acoustic path.
By providing a temperature probe having an extended sensor region, for example by using a number of point sensors with a known spatial interrelationship or, more simply, by employing a length wire of known temperature versus electrical resistance characteristics, then the average temperature of gas within the acoustic path traversed by the acoustic energy can be monitored. In this manner a temperature measurement can be made which more accurately reflects the temperature of the actual gas through which the emitted acoustic energy propagates.
The length of wire may be provided with one or more bends so as to form, for example, a wire loop, spiral or zigzag pattern, so that the total length of wire employed as the sensor region is longer than the acoustic path length. This leads to an increased electrical resistance change per degree of temperature change and so reduces any signal amplification requirements.